1. Field of the Invention
The present invention relates to apparatus for the photographic reproduction of transparent documents in black and white or in color, such as enlargers for amateur or professional photography and more particularly apparatus providing half-tone or continuous-tone negatives for four-color printing.
2. Description of the Prior Art
The optical principle of these apparatuses is that of a photographic enlarger. The optical elements comprise a light source, a parabolic mirror condenser and interchangeable lenses of various focal lengths.
According to the type of apparatus, the object plane is fixed and the image plane is movable or the object plane is movable and the image plane is fixed. Each of these two types have advantages and drawbacks. In the case of the fixed object plane, the image plane is moved, the object plane being at man's height. The documents and the masks are in a fixed working plane at the height of a man, which facilitates manipulation. On the other hand, according the maximum separation which is thus imposed between the two object and image planes, field angles of 60.degree. may be reached.
In the case of a movable object plane and of the fixed image plane, the field angle is generally less than 60.degree.. On the other hand, the documents are sometimes scarcely accessible, which renders manipulation of the masks difficult.
Apparatus providing negatives for printing also comprise an auxiliary exposure source designed to flatten the contrasts at the level of the film to be printed. This auxiliary system may be constituted by a device scanning the image plane before taking of photograph by means of a tubular source or by an annular source or a ring of optical fibers placed around the objective.
It is an object of the invention to provide an apparatus enabling increase of the maximum useful sizes to reach, for example, the size of 20.times.25 cm for the original and 50.times.65 cm for the reproduction, and offering the possibility of photometering any point of the image plane when the principal and auxiliary sources are simultaneously illuminated.
In the course of the description which follows, the photometric problems relating to apparatus of the enlarger type will be recalled.
It is also an object of the invention to provide an apparatus which has uniform lighting in the plane of reproduction.
It is possible to summarize the conditions indispensible to this end in the following manner:
As regards the source, the latter must be assimilated to a plane source and placed perpendicularly to the optical axis of the system and have preferably a uniform luminance whilst radiating according to the Lambert law.
As regards the condenser, the latter must be of large dimensions to cover the maximum sizes imposed, to correct the aberrations well, in particular spherical and chromatic aberrations, in order to obtain strictly in the plane of the pupil of the objectives an excellent image of the source.
As regards the adjusting system for the sizes of the image of the source in the plane of the pupil of the objectives, it is necessary for the latter to permit selection at will according to the characteristics or the defects of the original, an illumination of the so-called semi-directed or directed, diffused light type, the image of the source in the plane of the pupil having to be respectively greater than, equal to or less than the diameter of the objective used.
It is necessary to provide a corrector screen for the influence of the inclination .alpha. of the useful beam to the axis of the system which is single if possible, whatever the objectives and enlargements required and which operates at normal incidence for all the field points in order to facilitate calculation and measurement of the absorption at different points of the corrector screen of the cos.sup.4 .alpha. factor.
As regards the auxiliary illumination source, the latter must produce a uniform illumination taking into account the cos.sup.4 .alpha. factor, whatever the position of the reproduction plane and enable photometering of the plane when it radiates simultaneously with the principal source.